Light

Most of what we know about astronomy has been learned by studying light, so
it is important to understand how it behaves.
Light is a form of electromagnetic radiation, vibrating electric and
magnetic fields moving through space. The Greek letter lambda ()is used for the distance between crests, or the "wavelength".

Wave properties:

1.
Diffraction

When light encounters a barrier, such as a slit, its path bends and
it can illuminate areas behind the slit that are larger than the width of the slit. Here
are water waves at a breakwater; diffraction causes the semi-circular
pattern behind the breakwater (to left, from
http://www.exploratorium.edu/) and
diffraction in action to right.(animation by G. Rieke)

2. Interference

How interference works: the black wave is the sum of the blue and
green ones. When the blue one is "in phase" with the green, they interfere
constructively and the black one is larger than either. When the blue one is "out of
phase", they interfere destructively and cancel each other out; the black wave
vanishes.(animation by G. Rieke).

Rather than drawing the curvy lines for a wave, sometimes we just
draw a straight line for the wave crest seen from the top.

Here is how the interference animation transfers to this version. (by
G. Rieke).

We will show the kind of experiment used to see light
interference after the discussion of diffraction and particle behavior.

Particle properties:

1. Discrete energies

Photons have specific, discrete energies; the
shorter their wavelength, the greater the energy (to be discussed below).

2. Isolated arrival times

Here is a movie of a comet, taken with a device that amplifies low
light levels and shows every photon. Note the grainy appearance, due to the detection of
individual photons from the sky. This appearance results directly from the discrete energy
and isolated arrival times of the photons.

If we shine photons one at a time into the box below, we will detect
them as discrete particles, each one at a specific position against the back surface.
However, if we collect a large number of photons, even one at a time, they will distribute themselves in the
"dark" areas and avoid the "light" ones. The "dark" areas
are where the positive wavefronts overlap from the two slits letting each photon through.
Thus, they result from the combination of diffraction and interference. Perhaps
the most curious part is that each photon must pass through both slits!
(since we get the pattern even they go through the slits one at a time). This experiment is called "Young's fringes" after the first scientist to do it.
Experiment for yourself at
http://www.animatedscience.co.uk/blog/wp-content/uploads/focus_waves/rippletank__ripple.html

If you find this experiment troublesome -- well, so does everyone elseHowever, it is known to
be a basic aspect of the behavior not only of photons of light, but of all
fundamental particles such as electrons and protons! This topic has become an
entire branch of physics called quantum mechanics.

Some Basic Properties:

The speed of light, c, is a universal constant

c = 3 x 108 meters/sec or c = 3 x 105
kilometers/sec

c = times

where is wavelength and is frequency. The frequency is the number of wave
crests that pass by a given fixed point per second. From this equation, if we know the
wavelength we can compute the frequency, or if we know the frequency we can compute the
wavelength.

We call a particle of light a photon, so it travels at speed c, obeys
the above relationship between wavelength and frequency and it also follows this relation:

E = h times = h = h times c divided by = hc/

where E is energy, h is a constant called Plancks constant and is frequency.
Thus, if we know the wavelength or frequency, we can compute the energy.

The "Kelvin" temperature scale sets 0
at the lowest possible temperature, where the atomic motions are "stopped"(to the limits set by quantum mechanics). The speed of motion of molecules is proportional to
the square root of T(Kelvin) divided by the mass of the molecules.

We compare hydrogen (yellow, mass = 2) with oxygen (blue, mass = 32)
to the left. As the temperature goes up, the speed of the molecules increases (especially
for the low mass ones) and they hit the walls of the box harder. Thus, their force on the container walls (the pressure) increases. (animation by G. Rieke)

Examples: 0o Kelvin = -459o F;

273o K = 32o F = 0o C

Note that Centigrade (or Celsius) degrees are the same size as Kelvin
degrees.

Fahrenheit degrees are only 5/9ths as large.

As objects get hotter, they emit more
and at shorter wavelengths.(animation
by G. Rieke) The curves as shown to the left are
called blackbody curves  they represent the distribution over wavelength of
the energy emitted by a hot object whose surface would appear perfectly black if it were
cool. Many astronomical objects radiate energy almost as though they were blackbodies.

We now discuss the basic "radiation laws" that describe te behavior of
blackbody radiators in physical terms:

The rapidincrease of energy with
temperature is why we can display only a limited range of temperatures in a
"linear" graph like the one above. We can do better with a
"logarithmic" graph where the divisions are in factorsof
ten:

where F = number of photons produced by the source and r = objects
distance

This equation follows from the area of a sphere centered on
the source, . If the brightness fell off with distance slower than
, photons would have to be created in empty space to add to those
from the source. If it fell off faster, then photons would have to disappear into empty
space. (From Clem Pryke, http://find.uchicago.edu/~pryke/compton/flyer.html)

Light is attracted by gravitational fields.

According to Einstein's theory of relativity, light is attracted by
gravitational fields. This effect was originally confirmed by observing the apparent shift
in the positions of stars as their light grazed the limb of the sun, but is now observed
in many other situations.
Animation from Firstscience.com, http://www.firstscience.com/site/articles/einstein.asp